I am wondering if the piece of metal is a remnant of the original trunk floor.
Attached are some pics I have saved over the years. Note how the trunk pan has a vertical flange that turns up and is spot welded to the outside of the tail panel.
Very odd how the 70-71 parts books list the cap as 14-86-35 on the exploded diagram but when you go to its part number section (at least my PDF version) it goes from from -32 to -38 skipping the fuel cap!
The '72 book has a part number listing for the caps as does the '73 and '74 parts books...
Maybe this will help. Start reading at Post 203
https://forum.e-bodies.org/your-restoration-project-roseville-moparts/10/1970-fe5-cuda-build/200/msg342269#msg342269
This link at Moparts lists the Pan For Gold article as having come from Mopar Action Dec 1993 Vol 5, Num 4
Scroll down to the restoration category
https://www.moparts.org/Tech/MoparAction/Article.html
It appears it was from Mopar Action. Link bellow lists it as from their Issue #7 of reissues.
Not sure where I originally snagged it from. I am guessing from Moparts way back when.
https://www.brianschreurs.org/www.coltranet.com/publish/techspl.html
Great information!
When you remove the rivets, are you having to drill a hole for the screws? Self tapping screws or are you running a tap in there? Loctite when done?